Principles of Public Health Exam 1

Herd immunity threshold is the proportion of people in a population who must be immune (through vaccination or previous infection) to stop sustained transmission of an infectious disease. When enough individuals are immune, the disease cannot consistently spread from person to person, and transmission declines. This threshold is tied to how contagious the disease is, described by the basic reproduction number; the approximate rule is that the threshold equals 1 minus the reciprocal of R0. So, more contagious diseases require a higher share of the population to be immune, like around 60% for moderate spread and about 75% or more for highly contagious pathogens. It's a population-level concept aimed at preventing ongoing transmission, not a fixed population size, vaccine production targets, or the exact duration of immunity—though waning immunity and vaccine effectiveness can influence whether the population stays above the threshold over time.